OAKLAND, Calif. (AP) — Oakland police used tear gas and “flash” grenades Saturday to break up an estimated 2,000 Occupy protesters after some demonstrators started throwing objects at officers and tearing down fencing. There were at least 10 arrests and no reports of serious injuries.

Officer Jeff Thomason said police started making arrests when some in the crowd started throwing objects at them. Police declared an unlawful assembly after marchers tore down perimeter fences at the vacant Henry Kaiser Convention Center.

Oakland officials say about 250 people were in the group when the protest started on the city’s streets around noon, with demonstrators threatening to take over the vacant center. The crowd had grown to about 2,000 hours later, KCBS-AM reported.

The group was being monitored by dozens of Oakland police officers, as well as officers from nearby police agencies. Police began firing tear gas and flash bang grenades as things began to get out of hand.

The demonstration comes after Occupy protesters said earlier this week that they planned to move into a vacant building and turn it into a social center and political hub. They also threatened to try to shut down the port, occupy the airport and take over City Hall.

In a statement Friday, Oakland City Administrator Deanna Santana said the city would not be “bullied by threats of violence or illegal activity.”

Interim police Chief Howard Jordan also warned that officers would arrest those carrying out illegal actions.

Oakland officials said Friday that since the Occupy Oakland encampment was first established in late October, police have arrested about 300 people.

The Occupy Wall Street movement, which denounces corporate excess and economic inequality, began in New York City in the fall but has been largely dormant lately.

Oakland, New York and Los Angeles were among the cities with the largest and most vocal Occupy protests early on. The demonstrations ebbed after those cities used force to move out hundreds of demonstrators who had set up tent cities.

In Oakland, the police department received heavy criticism for using force to break up earlier protests. Among the critics was the mayor, who said she wasn’t briefed on the department’s plans.